how to beat blocks when try to initiate speech ?

how to beat blocks when try to initiate speech ? i speak almost perfectly with relatives, doctors, on the phone and in circumstances that i can take my time and the topic of conversation is predictable(like in a work context/exam etc). but i many times block a lot when i try to initiate conversation out of the blue like small talk with a coworker, say a joke with many pauses , "color" my voice etc.