There are loads of questions to sit down with the bride & groom & ask in advance.
Brides fantasize about this day from the point where they're about 2' tall... this is their fantasy day and they have a lot of ideas about what it will look like. It's YOUR job to capture that "fantasy" wedding.
However... as all things photography... the gorgeous photos don't happen by accident. It's all planning and a willingness of not just the photographer... but also the subjects... to recognize that they have to do their part to make the photos work.
Throughout the day there can be stress but keep a good mental attitude to keep the moods up ... work with the subjects to keep them thinking happy thoughts, fond memories, humor, etc. If they are in a good emotional place then it comes through in the photos. This means YOU don't want to be an emotional wreck worrying about the photos either.
I've done a LOT of weddings and most of it is not technically difficult... it's just anticipating "what's coming up next" and "what settings should I be used to capture that?" The camera was always ready for the "next shot".
Let them know they'll want to set aside time for photos at a few points during the day.
You'll also need to know their plan for the day... basically the flow (where, when, who, etc.)
Where is the wedding?
Is it in a church?
Have you seen this church and what's the lighting situation at the church (btw, all churches have poor lighting)?
When and where does the bride plan to get ready?
Some of the question typically need to be asked without the groom present because superstitious brides don't want teh groom to know anything about the dress. So...
Will there be a veil?
Will the dress have a train?
Brides sometimes want the "getting ready" shots... these are often shots with the brides mother helper her by putting final touches ... putting the veil on, helping with hair or makeup, helping with flowers, etc.
If there is a train on the wedding dress there are some techniques on how to handle it. We would have the bride face away from the camera, take a few steps, then turn to face the camera and this causes a twirled train in front of the bride. However... we also usually showed the maid of honor how to fluff the dress to get the fabric to lay well for the camera. (The bride will probably NOT want the groom to know anything about the dress... keep that in mind.)
99% of the time... you aren't allowed to use "flash" during the "official" part of the ceremony. The "official" part typically begins at the point where the priest/minister asks "who gives this woman to be married?" So the walk down the aisle... flash away. But once the bride arrives at the alter and the priest/minister asks that question... no flash from then on.
But ASK... sometimes the minister doesn't care (they usually do NOT want you to do anything that interferes with the flow of the ceremony). Meaning... you probably can't stand at the alter to take photos during the ceremony, but you CAN stand off to the sides, etc. using a long lens and no flash (so low-focal ratio lenses are your friend).
You'll want to ask about the details of how they plan to process up to the altar (assuming a church wedding). In my early days, the groomsmen escorted the bridesmaids up the aisle... but that doesn't happen often these days... usually all the groomsmen are waiting at the altar with the groom.
How many members in the wedding party? You'll want to know that.
Once guests are seated, the last people to be seated are typically the parents of the groom and the mother of the bride (often the father of the bride is walking his daughter up the aisle). You'll want photos of all of that.
Tip: YOU will stand roughly 1/2 way up the aisle and ask a guest to make a small space for you to step out of the aisle (into the pew).
PRIOR to the ceremony (as they are lining up to get ready), let them know where you'll be and tell them that when they see you step out of the pew to get their photo... to have them PAUSE for a moment... until they see YOUR FLASH FIRE TWICE (you'll take TWO photos of EVERYTHING) and then you step aside so they can continue.
Also... some coaching on how the bride/bridesmaids carry the flowers. You can often get the "nervous" look with the arms having a sharp angle at the elbows as they hold the flowers up near their breasts... you want a "gentle" and "relaxed" angle with the flowers down low. Shoulders relaxed a bit (tell them to take a few deep breaths and let their shoulders settle).
You'll have a much better "keeper" rate on those shots if they aren't moving when you shoot (so the pause trick works nicely).
When the bride comes up the aisle, you'll not only get those shots... you'll also turn around to get them arriving at the altar. Remember you can get a few last photos with flash until the ceremony "officially" begins and then quench the flash.
You'll want to know what special parts of the ceremony will be performed. For example... once upon a time, it was popular to have a "unity candle" (I don't see this too often anymore) ... the parents of the groom lit a side candle, the parents of the bride lit another side candle, the bride & groom used their respective parent's candle to jointly light the main "unity candle". Anyway... the idea is that if they are including any special elements in the ceremony, you'll want to know so you can get those shots.
The blessing of the rings is usually important and certainly the placing of rings on each other's fingers... and ultimately the "first kiss". You'll want to be in the middle of the aisle for those shots.
You'll want to know: Will there be a reception line and, if so, where? At the church? At the reception?
Typically as the bride & groom exit (shots you'll also need to get), IF there is a reception line, then formal photos happen after. But if the reception line is at the reception, then you can start with the formals once the guests have left.
Those who need to stick around include the wedding party and parents and/or grandparents and immediate family. ASK the couple if there are others who they want included. Usually it's just those I just listed but occasionally there's some close friend who wasn't also in the wedding party already.
When you start to shoot the formals... a few things to keep in mind...
#1 Everyone looks better with shoulders roughly 45° to the camera. Standing with shoulders "square" to the camera isn't usually as flattering. Bride & groom typically center and everyone angled toward them.
#2 No fig-leaf poses (guys shouldn't use their hands to cover the crotch... it's just not a good look. They can let their hands hang to the side... they can put hands in one pocket (often with the thumb out), etc. etc. etc. but avoid the "fig leaf" pose.
Take the photos of the bride & groom with the BRIDES PARENTS FIRST. That's #1 on the formals list. This is because they are the traditional "hosts" of the wedding reception and if the guests are waiting at the reception hall, the parents need to be finished so they can head to the hall. It's also a good idea to get any grandparents shots.
After brides parents, brides parents & grandparents, then brides parents WITH grooms parents... you can dismiss (with your thanks for their patience) the brides parents so they can go to the reception. Now shoot the couple with the grooms parents & grooms parents & grandparents. Now you can thank & dismiss them.
Now you can shoot the wedding party with bridesmaids & groomsmen.
Finally you can shoot the bride & groom.
If they hired a limo, you probably want photos of them getting into the limo and a final shot of them "in" the limo.
Congratulations, you've mostly dealt with the "wedding" part of the day.
Some weddings have quite a delay between the wedding ceremony and the reception. If this is YOUR wedding, you can consider shooting locations for the time between. We sometimes used a scenic park. Just make sure that if you select a scene ... that you wont run afoul of any rules.
You want to beat the bride & groom to the hall.
Depending on how much time you have... SHOOT THE CAKE ... this is not "the" shot for the cake... it's a "safety" shot. I've had weddings where ether (a) the cake wasn't supported well enough and it fell or the more likely (b) children couldn't keep their fingers out of the frosting. You want to get the cake before either possibility can occur.
ULTIMATELY the better cake shot is taken after the wedding party arrives... you'll borrow the brides bouquet and possible a few of the bridesmaids bouquets and you'll "dress" the table around the cake with the flowers for a better shot.
Make sure you get the shots of the wedding party arriving.
At the reception, you'll need to capture a few shots of the bride & groom kissing (as the guests ring the glasses).
Keep in mind the best man and maid of honor traditionally offer a toast and you'll need those shots (with bride & groom).
Any shots involving tables of guests look a LOT better BEFORE people eat than AFTER people eat (at which point the tables are a mess). But if you are asked to take shots of tables after... don't be afraid to clear some of the clutter off the table (everything "counts" in photo composition and cluttered messy tables wont do).
If the wedding has a buffet line, YOU will get in line after the wedding party (never before the bride/groom or their parents) ... but before the rest of the guests... REGARDLESS of what order the tables are called up to eat. There's a good reason for this... The bride & groom wont get much free time but when THEY are eating YOU need to be eating. When THEY are finished, YOU need to be finished.
If the wedding is at a photogenic location (stately manors, golf clubs, etc.) then it's often a good opportunity get the bride & groom into a gorgeous landscape for some shots.
Discuss (long before the wedding) the idea of them spending about 20 minutes away from the guests to do this at some point during the reception. Some brides have ideas about shots they want (I had a couple that wanted a shot AT SUNSET (which meant I used my astronomy software to find the precise time & location and pre-scouted to set up the shot.))
At the reception there will be typically be a few key dance shots you need to get...
#1 the couple's "first dance" as bride & groom typically opens the dancing part of the evening. You'll need several shots of that.
One tip on dance shots... as couples dance arm & arm, typically one arm is around each others' backs and the other hands are clasped... I typically "tap" the hands that are clasped and ask them to "pause" for just a moment to get the shot. If they are swaying (and not paused) there's a high chance you'll get a blurry shot.
For the bride & groom, the dance floor will be empty and you can get several full-length shots as well as some "half shots" (waist-up).
For everyone else, it's typically "half shots".
There will also typically be a "father daughter" dance and also usually a "mother son" dance. You need to capture those as well.
Also capture each of the bridesmaids with groomsmen dancing.
There may be a bouquet toss and a garter toss.
For the bouquet toss, take some control here... position the line of eligible bachelorettes where YOU need them with the bride in front so you can get the composition you need (they will all follow your lead when you tell everyone where to stand.)
For the garter... YOU will grab an empty chair from the nearest table and put it in the middle of the dance floor where YOU want it to be (trust me... everyone will follow your lead again). You will position this so you have a place to shoot with a decent backdrop.
The garter toss is similar to the bouquet toss... tell the eligible bachelors where to stand so you position the groom and the rest of the guys for good camera composition.
And then typically the winner of the garter has to put it on the leg of the winner of the bouquet ... and then dance. Al shots you'll get.
Cake time:
Once again, you direct the setup... stand to one side of the cake table so you compose the cake with the bride. The groom will be "just behind" the bride with his hand reaching around and gentle resting his hand on the bride's hand to "assist" in the cut. This gets everyone composed in the frame nicely.
They'll then feed each other a piece of cake (often trying to smash it into each other's face) and THEN they'll kiss (often with frosting still on their faces). Get that shot.
At this point you've covered most of the necessary shots.
I never leave the reception without asking the bride & groom if there are any shots they want... shots including special guests (often they'll have childhood friends or family members that don't live nearby, etc, etc. )
Make sure you have appropriate gear (cameras, lenses, flashes, etc.) and lots of battery power, memory cards, etc. If you have someone who can assist you then that can be a big help (holding off-camera flash, holding reflectors, etc.) If you have any concerns about your equipment, you can certainly rent gear... but rent it to arrive far enough in advance to (a) make sure shipping delays don't ruin the event and (b) you have time to test the equipment to make sure it's all working well, etc.
LensRentals suggests you schedule the rental to start a few days before so that if equipment arrives damaged from shipping, they still have enough time to over-night you replacement gear.
Good luck!