How do you play video in unity?

See this: http://unity3d.com/support/documentation/ScriptReference/MovieTexture.Play.html

Is this what you are looking for? (from http://unity3d.com/support/documentation/Manual/Video%20Files.html)

 function Update () {
 if (Input.GetButtonDown ("Jump")) {
     if (renderer.material.mainTexture.isPlaying) {
         renderer.material.mainTexture.Pause();
     }
     else {
         renderer.material.mainTexture.Play();
     }
 }

}

i think the best answer for that is Handheld.PlayFullScreenMovie(VideoPath);

Make a Folder Named StreamingAssets inside of Assets Folder and Place your video in that folder. eg. That Video's name is Test.mp4

now all you have to do is to Write this Code

Handheld.PlayFullScreenMovie("Test.mp4");

@dalekandjedi

After Unity 5.6 MovieTexture will no longer be used. With new Unity 5.6, VideoPlayer and VideoClip API should be used to play videos. Here's a very nice tutorial for that.

http://justcode.me/unity2d/how-to-play-videos-on-unity-using-new-videoplayer/

I know this was asked in 2012, but for anyone searching in 2015, you can use : Handheld.PlayFullScreenMovie() to play a video file on mobile.