How actors remember their lines

Boing Boing: How actors remember their lines This is quite an interesting one:

According to the researchers, the secret of actors’ memories is, well, acting. An actor acquires lines readily by focusing not on the words of the script, but on those words’ meaning – the moment-to-moment motivations of the character saying them – as well as on the physical and emotional dimensions of their performance.

I think this is important because I know when memorising song words, I do exactly the same thing. How do I know this? I often sing the wrong words, that have the same or a similar meaning. For instance, well I can’t think of anything right now, but I do it all of the time.