Why was ricky tarr holding a gun on people sending a message in tinker tailor soldier spy?

In the book, Ricki Tarr who is posted AWOL and thought to have defected by the Circus is sent by George Smiley and Peter Guillam to France as part of a ploy to trap the mole. Ostensibly resurfacing, Tarr is tasked to send a signal from Paris that he has information vital to the safeguarding of the organisation. Knowing this will provoke the mole into action, Smiley and Guillam then set up surveillance on a safe house where the mole will meet with his Russian contact.

The details are that Tarr goes to Paris and enters the unlocked car of Steve Mackelvore, the Circus member attached to the Paris Consulate, which is stationary in a traffic jam. Tarr makes Mackelvore take him to the consulate to send the signal. Tarr must use the gun to force Mackelvore and the cipher clerk "Ben" to carry out his instructions because he has been identified to Circus personnel as AWOL and believed to be a defector. Tarr waits with them (with the gun stuck into his pants to let them know he is serious) whilst the signal is sent, the mole is caught and a signal exonerating his actions is actions is returned.

The scene in the series of Tarr pointing his gun at the two persons (Mackelvore and Ben) is a contraction of the action which takes place in the book.