After Joseph Stalin was acclaimed as leader of the CPSU in 1929, Pasternak became further disillusioned with the Party’s tightening censorship of literature. Still unwilling to conform, Pasternak remained a close friend of Anna Akhmatova and Osip Mandelstam. Mandelstam recited his searing indictment of Stalin, the Stalin Epigram, to Pasternak soon after its composition in late April 1934. After listening, Pasternak told Mandelstam, “I didn’t hear this, you didn’t recite it to me, because, you know, very strange and terrible things are happening now: they’ve begun to pick people up. I’m afraid the walls have ears and perhaps even these benches on the boulevard here may be able to listen and tell tales. So let’s make out that I heard nothing.”
On the night of 14 May 1934, Mandelstam was arrested at his home based on a warrant signed by NKVD boss Genrikh Yagoda. Devastated, Pasternak went immediately to the offices of Izvestia and begged Nikolai Bukharin to intercede on Mandelstam’s behalf.
According to Olga Ivinskaya, Pasternak was deeply upset by Mandelstam’s arrest. He was concerned for his friend but he also worried that he might be blamed for fingering Mandelstam to the secret police. Ivinskaya writes that Pasternak “raced frantically all over town, telling everybody that he was not to blame and denying responsibility for Mandelstam’s disappearance, which for some reason he thought might be laid at his door.
Soon after his meeting with Bukharin, the telephone rang in Pasternak’s Moscow apartment. A voice from The Kremlin said, “Comrade Stalin wishes to speak with you.” According to Ivinskaya, Pasternak was struck dumb. “He was totally unprepared for such a conversation. But then he heard his voice, the voice of Stalin, coming over the line. The Leader addressed him in a rather bluff uncouth fashion, using the familiar thou form: ‘Tell me, what are they saying in your literary circles about the arrest of Mandelstam?’” Flustered, Pasternak denied that there was any discussion or that there were any literary circles left in Soviet Russia. Stalin went on to ask him for his own opinion of Mandelstam. In an “eager fumbling manner” Pasternak explained that himself and Mandelstam each had a completely different philosophy about poetry. Ivinskaya writes that he “went on for quite a time in this vein. Stalin gave him no encouragement whatsoever, not interjecting, or uttering a sound of any kind. At last B[oris] L[eonidovich] came to a halt. Stalin then said, in a mocking tone of voice: “I see, you just aren’t able to stick up for a comrade,” and put down the receiver."
— Wikipedia article on Boris Pasternak.