to suddenly start paying attention to someone, because they have done something surprising or impressive:
If Maria succeeded, then everyone would sit up and take notice.
WORD CHOICE: WORD CHOICE: sit, sit down, sit in/on, seatYou usually use sit down rather than sitto say that someone moves into a sitting position• Everyone sat down to listen.You use sitwhen you mention where someone sits down• She sat next to me. • Where shall I sit?!! You sit on or sit ina chair, depending on whether it is flat and simple or soft and comfortable• We sat on barstools. • He sat in his favourite armchair. You sit on flat things such as a bench, the floor, or the grass. You sit ina room, a corner, long grass, a tree, or a seat in a car• I get travel sick when I sit in the back.!! To tell someone to sit down, say'Sit down', 'Have a seat', or in very formal situations,'Be seated'. You usually only say'Sit!' to a dog.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.