I am having the worst luck trying to remove the background of a 1 minute and half clip. I have footage of a gentleman talking and I would like to remove him from the scene (there is a boring white wall with curtains behind him) and replace it with an image. I did not unfortunately use a green screen so I am stuck! Any advice/tips would be greatly greatly appreciated.
It would be easier to give you some direct suggestions if we could see the shot. Just a single frame or a screen grab from AE would do.
The easiest way, if your footage is compatible, is to create a procedural matte. You do that with channels, levels, and or keying. The hardest way is by rotoscoping. Good, clean footage with a limited amount of detail works well with AE's automated RotoBrush tool. The more detail you need to mask out, the harder it is to rotoscope. Rotoscope or roto is the procedure of hand painting a matte.
Post a frame and we'll try and give you some pointers.
Read this thread: Procedural Mattes
Look at the whole thing. It will give you an idea of one way to pull a procedural matte. Check out the links at the end of my post.