How might it compare when deciding to run Curse of Strahd for existing characters that may have come from other adventures vs characters who are made for Curse of Strahd?

With existing character you have the option to start the adventure at higher levels. This can help with jumping to later points in the adventure. It will require different scaling on world content to keep the challenge meaningful. I ran a campaign with fresh characters, so I can’t offer a direct comparison.

When making characters specifically for Curse of Strahd, do certain types of characters or party concepts enjoy the campaign more than others?

Characters who are driven by justice are important. With a few characters in the party who are strongly motivated to either kill or join Strahd one can ensure the campaign stays on tracks. Characters who are tied to concepts of life and death, or the balance of nature will be more tuned into what is happening in Barovia.

Is conceptual guidance from the DM helpful in this case, or not?

Coordinating with the DM to make the character will lead to more touch points for the player. At my table the characters who were better coordinated with the DM had more story moments built for them on top of the modules content.

What are some good ways to bring the party together at the beginning?

Running the Death House is a great way to get the party on the same page. Alternativly have the party encounter the Vistanti Fortune teller early on. The Death House helps prove out the malice of the world and show the party that they can change things in their favor…kinda. Encountering the Fortune Teller will be a strong direction setting exercise. The villagers in the Village of Barovia can mention her as someone they seek out, this can help key them in to her existence.

How can you bring in new PCs if a character dies?

Keep in mind that characters under level 5 can make a deal with the dark forces of the land to be revived.

For bringing in the new character – if they are above level 3 perhaps they have already been in Barovia adventuring. This gives the DM a chance to seed the character with lore the party hasn’t found yet. They can be a few steps ahead of the party in their adventure. We had a character join at level 2 in Death House. They were in a bedroom that party hadn’t opened yet. Another joined at level 3 riding in on a wagon with the Vistanti. Once ran in with a pack of wolves during a fight with dark druids at level 6. One joined at level 7 for a dinner party, they were an old friend of an existing party member. Another helped save the party from an after festival kerfuffle with the Mayor of Valaki.

Try to have new character intros put them into existing players good graces. If they are a vetted old ally or immediately helpful the other character / players will be more likely to accept them into the party.

As a DM, keep this in mind because if you coordinate well with an existing player this can be a way to place an imposter with the party.

Try to keep the party balanced as you go, so their average motivation direction keeps the plot moving forward.