How are actors pay determined?

i know it depends on the box office to...but how will they know they'll get paid that exact amount, because jim carrey knew he was gonna get paid 20 million for cable guy...how did he know that....like robert downey jr.....he star in the movie iron man...and the movie made little more over 300 hundered million.....how much did he get paid...and for titanic...starring leonardo dicaprio...the movie made 600 million in the box office..but leonardo only got paid about 2 million...why did he get paid soo little if the movie made a lot..and i know u have pay the director..the producers...and script writers..ect...but i know there will be a little more million in the box office left.....hope i explained it right...thank u