Let’s face it; Clive is terrible at impersonating Luke. I made a helpful chart comparing their behaviour:

So: How in the world did he get away with it for even an hour?

The answer is easy.

He is the embodiment of what you would like your future self to be like.

Let’s start from a different perspective. If you were to face the future self of someone you know well, who would you trust as the judge over whether or not this future self is legit?

Of course, the person they are claiming to be. Who is Luke in this case. And Luke immediately trusted Clive.

So what Clive did, was, that he represented everything he thought might please Luke as a future vision, while still remaining vague enough to balance out the fact that he doesn’t know as much about Luke and his goals personally.

But just his appearance was influencing already. Clive, in his role as Future Luke, appears

-Well-dressed

-Smart & Illoquent

-Polite

Additionally, Clive was smart enough to not take a similar role to Luke at the Professor’s side, acting more as an “old friend“ to the Professor than what the actual mentor-student-friends relationship of these two actually suggests. But since it has “been“ ten years, he has no trouble setting up his own role at the Professor’s side:

Being introduced to your future self like this made it easier for Luke to accept his presence and not doubt him more.

Additionally, Clive also seemed to be trained or naturally talented in reading situations. Take this exchange for example, when Clive encounters Flora for the first time:

This situation was extremely risky, as the more people who know Luke personally and are in differing relationships with the real Like meet him, the higher the chance of being busted. Let me show you what I mean when I say that Clive was really lucky that Flora said what she said, and how he based his response off of that:

So yeah, conclusion, Clive is The Mentalist