Altrouge wrote:I understand what you're saying about the flare not supposed to move, so I was wrong; I'm not trying to make it realistic, just.??'believable', per se. Sorry if I'm asking too much, but since you've seen the clip already, do you know where the flare would start and in which direction it would move?
Well, lens flares can be caused by lots of different things, but generally the flare will line up in 3D space with the main light source in a scene. Look at a lens flare--see the line that goes through the middle and runs straight through the circles of light? Think of that line like a ray of light coming from the sun or whatever is lighting up your clip, and match it up to that light source. Or, if you have something highly reflective in the scene (like a sword), look at how light bounces off of it and line up the lens flare ray with that.
Lens flares can also be caused by light being refracted inside camera lenses or other transparent objects, though, so in those cases it's hard to say what will make them look the most believable. Just go for whatever looks best to you in that case.
Honestly, unless you have a very obvious, visible light source in your clip, or a big reflective object (like a humongous sword

), don't worry about realism with lens flares. But if you're specifically trying to make a ray of light race across a sword or something, just look for clues in the footage you have as to where the main light is coming from.