Some actions require attention though, for example reading. Wouldn't it be better to interrupt an action depending on the action type and players perception instead? Very unperceptive reader might not notice a quiet monster coming from behind until hit while alert eater will surely see a danger from afar.

I agree with ixi - the player should have to pass a perception check before noticing the monster if they're busy doing something else. The whole "monster sneaks up on you while reading a book" aspect gives the game a little more immersion and depth. (And teaches players valuable lessons about making sure they're in a safe place before doing certain things)

Maybe add a "The *foo* sneaks up on you!" message if the player fails a perception check to make it more obvious what happened?

Fixed.