While knowledge of the exhibits is essential, I'd suggest excellent people skills are paramount.
A few years ago, I had to sadly dispense with the services of one of our paid guides. Experienced...Yes, Encyclopaedic knowledge....Yes, Great passion for the museum....Yes, Pathological hatred of children...(regrettably) Yes.
Management could only take so many complaints from unhappy parents.
On the plus side, I enjoyed the story from the guide at the Australian War Memorial who always included a detailed spiel about his favourite exhibit, Flt Lt Brown's P-40, named "Polly", when conducting groups through the aviation hall.
One day he noticed that an older lady in one group seemed to spending a lot of her time examining the P-40.
He approached her, thinking she may be interested in learning more about the aircraft.
To the contrary, the ensuing pleasant conversation saw him learning even more about his favourite exhibit, courtesy of Mrs Polly Brown.
He later described meeting her as the highlight of his career.