Under conspiracy, what constitutes an agreement?

The most obvious type of agreement is express. For example, if Bob asks Sam, "Hey, Sam, would you like to commit a bank robbery with me after lunch?" and Sam responds, "Yes, Bob, I would surely enjoy committing a bank robbery with you." In the real world, it's usually not this clear. But that's okay, because you can prove an agreement existed to engage in a conspiracy by the conduct of the participants. For example, if Bob puts on a mask, holsters a gun, and tells Sam he's "headed to the bank for cash," and then Sam nods, puts on a mask, and hops in a car with Bob, even though Sam didn't expressly agree to commit a crime with Bob, Sam's conduct implies that he is agreeing to do so.

The other thing to look out for is the majority vs minority view implication. This usually comes up when an undercover cop is trying to lure a defendant into admitting their intent to perform a crime. For example, imagine that Sam is an undercover police officer and Bob is a suspected criminal. Sam and Bob plan to rob a bank and after loading up their car with guns and driving towards the bank, Sam arrests Bob. Under the majority view, Bob is not guilty of conspiracy, because Sam was only pretending to agree to commit a crime. Since Sam didn't actually agree to participate in the crime with intent of completing it (because he's a cop), the elements of conspiracy are not met because Bob is essentially acting alone. However, under the modern minority view, Bob is guilty of conspiracy, because only one person in the group need actually intend to follow through with the criminal plan, which means it doesn't matter that Sam is only pretending.