Best mode to use SB900 for fill flash?

[dndpaints]

Hello all,

So I have an SB900 speedlight that I've been using with my D7000 on occasion. I don't use it often, but when I do I currently only use it to bounce light and I have it in TTL mode (camera in Manual mode). I'd like to learn more about doing fill flash as I do a lot of outdoor portraits and I don't always have someone to hold a reflector. Is TTL still the preferred speedlight mode or should I try putting it in manual? Is a diffuser even worth using since they are so small??

Thanks in advance!