Oh boy, thank you so much for the compliments! That’s so kind! What an amazing question as well – I feel I’ve spent hours and nights discussing this with fellow writers. And for whatever my opinion’s worth, here are my two cents (I will try to keep it as short as I can):
As writers, first and foremost, I believe we must be good observers of behaviour. Understanding the motivation behind someone’s actions will allow you to replicate them accurately on paper. This is why for this project I messaged about 20-30 morticians on LinkedIn, 5 of which I ended up interviewing. We also did visits to a funeral home (and mortuary) and discussed the job with them at length. A lot of aspects of Daisy (and Nigel) have come from those discussions – mainly the incredible respect they have for their “clients” combined with a keen sense of humour that’s meant to keep the darkness of the job at bay. Research is crucial in creating authentic characters.
Secondly, internal conflicts, flaws, and contradictions make for richer characters. I’m not one to do extensive bios but I will write a paragraph that’s just a shallow dive into what the character’s flaws, conflicts, open wounds, and contradictions are. Even if our characters are largely sitcom tropes, we’ve tried to plant contradictions and internal conflicts for each. Sandra, for example, is the main originator of jokes and abuse in that staff room scene. But at the same time she is a mothering figure – i.e. she stayed up until midnight to make Daisy her favourite vegan cupcakes (a lot of morticians turn vegan after a few years on the job, which is another little factoid that came out of research
) and insists that she comes to the party. So, Sandra herself has these two facets of her personality that clash. She perceives this warm, mothering instinct as weakness in a male-dominated workplace and seeks to mask it with jokes at Daisy’s expense. This is partly also drawing on real world experience; I’m sure you’ve met people who are insecure and unhappy who look to take down others as a way of fitting in. If we ever get a series commission, this is something we would look to explore, expand, and potentially diagnose.
These are just two of the main ways I try to look at character, but this is already too long of a message, and I don’t want to keep rambling on.
Other tools I look at are forcing your characters to make choices, thus making them more active participants, surrounding them with other characters that trigger their internal conflicts or contradictions (e.g. who’s the worst person in the world who could cross paths with my character right now?), giving your characters opposing views of the world will organically put them in conflict with each other (they could simply have different opinions on how to deal with grief), etc.
Hope this answers your question and let me know if you have any follow-up questions. You can tell us about your characters and your approach, maybe specific things you’re struggling with? 
Eddie.