Bash and curl - pass variables to curl

I often call curl in a bash script. How do I pass variables that I’ve set in bash as arguments to curl’s command line option, say --data?

Below is an example without variables.

curl \
  --header "Content-Type: application/json" \
  --data '{
            "name": "Hello John Doe" 
          }' \
  http://example.com

Splitting John Doe into two variables, and passing them to curl makes the server display the greeting as "Hello $firstname $lastname".

# with variable - doesn't work. Variables are sent in request as is.
# Server displays "Hello $firstname $lastname"
$firstname="John"
$lastname="Doe"

curl \
  --header "Content-Type: application/json" \
  --data '{
            "greeting": "Hello $firstname $lastname",
            "firstname": "$firstname",
            "lastname": "$lastname",
          }' \
  http://example.com

I’ve come across two cases:

• If a variable is part of an already in a double quoted string, enclose in single quotes. See the value of key, greeting.
• if a variable is to be its own json value, escape the double quotes. See greeting. See the value of key, firstname and lastname

$firstname="John"
$lastname="Doe"

curl \
  --header "Content-Type: application/json" \
  --data '{
            "greeting": "Hello '$firstname' '$lastname'",
            "firstname": '\"$firstname\"',
            "lastname": '\"$lastname\"'
          }' \
  http://example.com

Thanks to Bernie Michalik’s post for the hints.