看热闹的人是rubberneck （rubber是橡胶的意思， 伸长脖子看热闹的人好像长了橡胶脖子）
看热闹的动词是look on或者watch the fun
举个例子：
Don't just stand there watching the fun! Hurry up and give me a hand.
别站在那里看热闹啊！赶紧给我搭把手。
凑热闹是 come along for the ride
He has booked two tickets for the concert tonight, but I just come along for the ride.