1. Act on - To take action because of something like information received. Ex : The police were ACTING ON a tip from an informer and caught the gang red-handed.
2. Act out - Perform something with actions and gestures.Ex :They ACTED OUT the story on stage.
3. Act up - Behave badly or strangely. Ex : My computer’s ACTING UP; I think I might have a virus.
4. Add on - Include in a calculation. Ex : You have to ADD the VAT ON to the price they give.
5. Add up - To make a mathematical total. Ex :We ADDED UP the bill to check it was correct.