Is there any way to position a video (of a smaller dimension than the background) in a room, and also, that the video is seen at the same time as the background? (As if, for example, there was a room with a movie screen and a movie was projected, for example. Thank you!
No, unfortunately AGS does not support non-blocking non-fullscreen videos at this time.
There's a plugin that can do this though:
https://www.adventuregamestudio.co.uk/forums/modules-plugins-tools/plugin-spritevideo-v0-9-3-sprite-and-video-render-based-on-direct3d-plugin/
The caveat is that it does not play any sounds in its current version.
For low-res games it may be worth doing a sprite animation instead, especially if it's just a emulation of video with limited number of frames, such as for a theater screen like in your question, or TV screen on background, and so forth..
Thank you. As always very effective and useful!