How/When to use a speedlight as fill flash?

[Derrel]

Here are three good short Strobist articles on using On-Axis fill flash. The top one is essential reading, IMHO.

Strobist: Ox-Axis FIll:Run-and-Gun Version
Strobist: On-Axis Fill: Run-and-Gun Version

Strobist: On-Axis Fill:Introduction
Strobist: On-Axis Fill: Introduction

Strobist: On-Axis FIll:Ring Fill Against Restricted Light
Strobist: On-Axis Fill: Ring Fill Against Restricted Light