Culture is a mirror that reflects back the combined personalities of the people at the company, and so I think the only tool that you have as a founder to define a culture is hiring and firing.
How should startups think about culture?

I was once talking to an engineering manager who used to work at Amazon — rather famously, a hard-driving culture. She was then working at a new company and talking about a situation where there was a bad production bug that her team was responsible for. Her team did not seem to be attacking it with proper vigor. They left at 6pm the day this bug came up, the same as every other day. Also, she didn’t feel like she could ask them to stay late, even though she wouldn’t have hesitated at Amazon to demand that.

Whether or not she should have, it’s interesting to note that the same person was acting completely differently because of the norms of the company she was in. That’s the power of culture. And the reason hiring and firing is not your only tool.

It’s very clear that people act differently in different situations. Personality is complex. We all have different sides of ourselves and are extremely aware and adaptive to the social norms around us.

Most people, most of the time, are reactive to the culture around them. Even if you’re a naturally friendly person, if you come to work every day and no one is smiley or chatty, you’ll eventually stopped being so as well (or leave). Same goes for how much effort is put forth. Most people trend toward the average. In a more effortful culture, they’ll work harder. In a more lax one, they’ll be more lax.

Where leaders (and in this case, that anyone who chooses to be a culture leader, but especially those in authority) can have a great effect is not just reacting to others, but demonstrating the culture they want. This may mean breaking your own habit and tendencies, which is non-trivial. But it is possible.

For example, I’m not the most outwardly friendly person. (Inside, I’m a teddy 🐻.) In the office, I’m often thinking deeply about a problem, which causes me to not fully acknowledge others in pass-by situations. I’ve realized this comes off as cold and/or unfriendly. Small gestures have an effect on how people feel in the moment. Small gestures, over time, get reflected and define a large part of how a place feels. So I’ve tried to change this tendency. I’m still working on it, but I know I’ve gotten better, and, though unmeasurable, I think it’s had some effect.

That’s a small example. There are larger examples — like holding people accountable. If you have a culture that doesn’t do this, moving to one that does is hard but possible (without replacing everyone). Or being more data driven. Or being more risk-taking. It’s really about asking the right questions and paying attention. And, of course, what sort of bad behavior you tolerate.

The culture of any organization is shaped by the worst behavior the leader is willing to tolerate. 
Gruenter and Whitaker

Here’s a good piece that references that quote and defines specific behaviors that make a difference, besides hiring and firing — a potent, but blunt and time-consuming tool for shaping change.