I heard Gehenom described different ways, and I think its not a set answer to each level. To a grade school child, “fire” is the most vivid description of punishment. To an older person, the “fire” is the shame of messing up and letting Hashem down. To the next level, its realizing that you missed an opportunity to reach a level that could have been yours, but now you can only have regrets.
But I have no doubt that there is an element of roasting over coals for having done an averah on purpose. Not with a wood and kerosene fire, but something that is gonna hurt like, well, hurt like H–L.
Suffice to say, its best to avoid sinning and spend our time racking up mitzvos, as the payout is MUCH better.
However, to answer P’chochma’s question, this is really something that needs a Rov’s guidence, but if I were asked by a not yet frum (or struggling) person what awaits them in the world to come, it would only be to focud on the reward part. You catch more flies with honey than with vinegar.
The “fire and brimstone” end of our religion is meant to keep us FFBs in line, and even that needs to be adminstered by a Rov or Rebbe. Its not a tool to be used by just anyone, so focus on the beauty of what awaits the faithful, not the punishment.