Very closely related: In what sense did Judas betray Jesus?
Then one of the Twelve—the one called Judas Iscariot—went to the chief priests and asked, “What are you willing to give me if I deliver him over to you?” So they counted out for him thirty pieces of silver. From then on Judas watched for an opportunity to hand him over.
What was the "opportunity" that he was looking for? And why did he have such difficulty finding an appropriate moment? As Jesus Himself noted in verse 55, He wasn't exactly a hard man to find.
Based on the following verses, as well as the linked question, it would seem that they wanted to arrest Him somewhere more private (or, at least, have some way to avoid making a scene when they did arrest Him):
Then the chief priests and the elders of the people assembled in the palace of the high priest, whose name was Caiaphas, and they schemed to arrest Jesus secretly and kill him. “But not during the festival,” they said, “or there may be a riot among the people.”
Why did they even bother with Judas in the first place? Why did they consider it worth the money? Was the entire point of hiring Judas the fact that he'd know when and where they could find Jesus alone? If so, did they really need Judas for that? Wouldn't there have been a lot of people who could've told them that?