How Can I Make Character Bounce When Changing Key Frames?

I have to confess I'm not experienced with videos (MP4s, Flv, etc.), but I can give you an idea to play with.

Import your video to the stage.

You will have put the video into a movie symbol.  I just tried what I'm going to recommend with the video itself, but no luck.  Below I'm listing steps I used to get the video into the symbol.

* Highlight your video and copy it (hit ctrlC if your using Windows).

* Now hit ctrl F8 to create a movie symbol.  An empty field will open.

* Paste your video into the empty field (hit ctrlV).

* Now you can delete the video from your timeline, open you library and drag the movie symbol with the video to the timeline. 

OK, for the trick to get it to bounce.  

* Hit F5 to elongate the timeline to make it long enough to play your video.

* Right click the keyframe with the video, then select create motion tween.

* Click on another frame in the timeline, leftclick on the symbol, and drag it to another location.

* Repeat the process in the preceding step at another frame.

* Repeat the process with as many frames as you need to get the affect you need.

Give it a try; I hope it works for you.

i assume you have a movieclip of your character talking, eg mc_talking.

copy and paste the mc_talking frames to a new movieclip (eg, mc_talking_bounce).  but you may not want to paste the 0 to x frames in mc_talking to frames 0 to x in mc_talking_bounce.  you may want to paste to frames 15 to x+15 and on frames 0-14 create your bouncing talking character.  on the last frame (x+15), use:

this.gotoAndPlay(15);  // so your character doesn't repeatedly bounce during speech