A quick thread comparing Soviet motivations for intervention between 1956 and 1989.
1) Poland. 1956. After Gomulka, whom the Soviets considered a nationalist, gained power and tried to force out pro-Soviet players from the leadership, Khrushchev threatened intervention.

He decided against this in the end. He claimed Gomulka promised that Poland would not abandon Communism. The jury is out as to whether this was the real reason, or whether he was just distracted by the events in
2) Hungary. 1956. Khrushchev went back and forth.

When the anti-Communist forces started to gain the upper hand, he decided at first against intervention but changed his position, seemingly because he was shown photos of lynchings of Communists and security agents.

Khrushchev talked at the Politburo about his worries about "losing" Hungary to the "imperialists" just at the time the imperialists (Britain, France, and Israel) were invading Egypt.

3) Czechoslovakia. The Soviets intervened after several months of the "Prague spring," when Leonid Brezhnev became convinced that his opposite number in Prague - Alexander Dubcek - was pulling him by his nose. This inaugurated the doctrine of "limited sovereignty".

A Soviet satellite was deemed "sovereign" only insofar as it pursued socialism (as the Soviets interpreted it). The one country that became very worried by this was China. Soviet invasion of Czechoslovakia directly precipitated the March 1969 border clash in the Far East.

The Chinese feared the Soviets might also try to impose their version of socialism on Beijing. (BTW, Brezhnev never considered this as far as we can tell).

Then the Soviets invaded Afghanistan. It's interesting to note that the Afghan leaders requested Moscow's intervention in March 1979 after an uprising broke out at Herat. The Soviets pondered the question, and refused. They worried about what this might do to Soviet-US detente.

But they changed their mind by December 1979, when they became convinced that the Communist leader of Afghanistan Hafizullah Amin was secretly plotting to turn towards the United States.

Then there was Poland. The Soviets pressured the Polish government to declare martial law. Historians disagree whether Moscow really did intend to intervene if Warsaw refused. Andropov reportedly quipped that the "quota of interventions has been exhausted" after Afghanistan.

The one person who just refused to intervene come what may was Mikhail Gorbachev. When things started to fall apart in Eastern Europe, he resisted calls for Soviet involvement. This was a number of reasons:

First, he worried that any such intervention would undermine his domestic reform and his foreign policy agenda. Second, he was supposedly averse to the use of force. And third, he didn't think that the regimes were saveable. Intervene and then what? Who will pay for them?

Putin is obviously not Gorbachev, and he is not averse to use force (as he has repeatedly shown). But the calculus of whether or not to intervene is very complex and entails a calculation of costs and benefits.

Broadly, the Soviets undertook intervention when they feared they might "lose" a client to the West. Putin's actions in Ukraine in 2014 betray a similar calculus.