Brad's given a pretty good explanation of interrupt/disturb.
As for "world" - well, the L is always pronounced. As for the R, US speakers will always pronounce it.
UK/Australian speakers have a different rule for pronouncing R in that situation, and to the listener it may sound soft or nonexistent (don't just drop it - we have a rule, remember). The actual pronunciation of the letter R also differs between styles - don't assume any are like the R that you know.