I think that “gravitas” means really the opposite of being a visibly light-hearted, humorous and happy-go-lucky person. Therefore you have to be able to switch the level of gravitas on and off, that’s the tricky bit. There are people who get noted for being able to make people laugh and sometimes, if they are not carefully, they become regarded as “clowns” who are not really to be taken seriously. On the other hand, too much of the opposite will mean you are seen as someone with gravitas, but maybe not someone with a great sense of humour, which could create distance in situations where you don’t want it.
A certain dignity in not being too open about one’s personal life is probably good – familiarity can breed contempt, and making sure that your use of humour is aimed at quality rather than quantity will do a lot to enable you to come off as someone with gravitas. Other than that it is something which grows with age, experience and the ability to show a certain classic style of speech, choice of language, dress and posture. But you can’t stand on your dignity the whole time. If you make a mistake and people laugh at you, better join in the joke rather than get offended or that will undermine gravitas quicker than anything else.
Certainly a serious demeanour going hand in hand with a genuine reputation for being ethical and having a flawless integrity, these are things which many of us aspire to and should aspire to. For many successful people, this is what they tend to have in common and for many this is what they will sum up as “gravitas”.
A former boss of mine suggested that I increase my level of gravitas in order to get on, he did not say how I should go about it, but I think to a degree I did it anyway. Now I am pleased to say I can still make people laugh, but I can just as easily get them to take an issue seriously. That’s what I wanted to achieve and that’s a target I recommend people to aim for whenever I hear them talking about gravitas.