Have you ever seen a flash button that have transitions between RollOver and RollOut? Well, this thing is done by creating an animated movieclip with a little bit of actionscript:

1. First, create a movieclip, and inside create your button design: a square shape with a text field or anything you want.
2. Edit this movie clip.
3. Create an animation with your shape and text inside this movie clip.
4. Then, insert a new layer in this movie clip, and insert this ActionScript in the first frame

   //all this code is inside the complex button so you can stick as many as you want
   //on the stage and all will work.. they all will have this code in it..
   
   stop(); // stop the movie clip from playing (stop button from growing, we want that when the mouse rolls over only
   
   this.onEnterFrame = function(){
   if(rewind == true){   //if rewind switch is set to true play backwards
   prevFrame();        // play backwards
   }
   }
   
   this.onRollOver = function(){
   rewind = false; //set variable (or switch) that decides wether ot not to play backwards...
   play();    // play this movie clip.. (grow the button(tween));
   }
   
   this.onRollOut = function(){
   rewind = true;    //set or rewind switch to true so it will play backwards...
   }
   

   

5. Now create a Keyframe at the end of the animation in this Layer, and insert a stop(); in the Actionscript panel.
6. And finally, save the document, publish it, and test it!

And here are the source files:

Flash 9 ActionScript 2 file: mclip.fla

Swf File: mclip1.swf