[General] A line in Sherlock (BBC)

A line of Sherlock
"God forbid the star witness in the trial should come across as intelligent"
Could anyone kindly explain what it means for me? I'll appreciate.

Here's the context.
Sherlock and John were in a cab, heading to the court to present as expert witness.
John: Remember what they told you. Don't try to be clever.
Sherlock: No.
John: And please, just keep it simple and brief.
Sherlock: God forbid the star witness in the trial should come across as intelligent.
John: Intelligent, fine. Let's give smartarse a wide berth.

lilylilly said:

A line of Sherlock
"God forbid the star witness in the trial should come across as intelligent"
Could anyone kindly explain what it means for me? I'll appreciate.

Here's the context.
Sherlock and John were in a cab, heading to the court to present as expert witness.
John: Remember what they told you. Don't try to be clever.
Sherlock: No.
John: And please, just keep it simple and brief.
Sherlock: God forbid the star witness in the trial should come across as intelligent.
John: Intelligent, fine. Let's give smartarse a wide berth.

Click to expand...

The phrase "God forbid" is usually followed by something that you really don't want to happen.

"I am going on a long drive tomorrow. God forbid it should rain really hard and I crash the car!"

Obviously, heavy rain and a car crash are two things you don't want to happen so you want "god/God" to "forbid" (prevent) them from happening.

In the dialogue you quoted, it is used sarcastically. Of course, you would actually want the star witness in a trial to appear clever (intelligent) but Sherlock is rather upset that John is telling him that he shouldn't seem overly clever (if you are watching the series, you will know that Sherlock can be brilliant but also very sarcastic and he doesn't deal well with authority! He can be, as John later says, a "smartarse"!) Consequently, he sarcastically says that it would be ridiculous to want the star witness to seem intelligent.

As you can see from the dialogue, there is an implied difference between "clever" and "intelligent".

Thank you so much for explaining so clearly.
I finally understand what Sherlock means.