Why do stars appear to twinkle?
Stars tend to twinkle for two main reasons: first, stars are very far away (the closest star is about 4 light-year from the Sun) and are therefore seen as point sources. Second, Earth has an atmosphere. Earth's atmosphere is turbulent, and therefore all images view through it tends to "swim". Therefore, sometimes a single point in "object space" is mapped to several points in "image space", and sometimes it is not mapped at all. Since stars are seen as single points, they sometimes seems brighter, sometimes even seems to disapear.
If you look at it in another planet of our solar system, it will depends on the planet's own atmosphere. If you look at stars on Mars, the atmosphere being very thin, the stars won't twinkle that much. Same for Mercury. On Venus, the atmosphere is so thick than you won't see anything apart from the atmosphere itself (if you are not crunched by the atmosphere pressure, by the way...).
Well atmospheric pressure doesn't affect us all that much, not nearly as much as the gravity from the planet itself. Venus and Earth have similar gravity so the pressure is what makes us feel like we weigh more than we actually do in a thick atmosphere. Air pressure itself varies day by day, hour by hour, even second by second which is why a barograph(like a barometer but it graphs the pressure instead of just measuring it) has significant and tiny variations in air pressure. And we have been able to survive high and low air pressure for thousands of years.
You couldn't survive the pressure on Venus (or the Temperature). The atmospheric pressure on the surface of Venus is equivalent to nearly 3,000 feet deep in the ocean. That would kill a human pretty quick, though there are animals that can survive those pressures. Elephant Seals and Sperm Whales for example.