Speech & Thought Bubbles
There are lots of ways to be creative with speech bubbles in your videos. It's not usually a "serious" look — speech bubbles tend to work well with humour.
Speech bubbles can also be used if the original audio is poor quality or has been lost altogether. You can either attempt to recreate the original speech using bubbles, or invent your own speech (as long as it's fairly obvious that this is what you're doing).
Thought bubbles are the same as speech bubbles except they obviously show hidden thoughts instead of spoken words. Instead of a triangular extension, use two or more smaller oblongs to indicate the thinker.
To create speech bubbles, you can either use the titling function in your editing program (if it supports shapes) or create the bubble in another application (such as Photoshop) and import it. The bubble itself is usually a simple oblong shape with a triangular extension pointing towards the speaker. The bubbles title/graphic is placed on a layer in the timeline, above the existing video.
Like most titles, speech bubbles should appear long enough for the viewer to read without rushing, then disappear.