For this entire article, assume the two sprites conversing are "Scratch Cat" and "Gobo".
The broadcast method isn't always the most efficient way to make sprites communicate many messages, but short conversations can be easily made using broadcasts and no variables or lists. The following script would go into "Scratch Cat".
when gf clicked say [hi!] for (2) secs broadcast [Gobo 1 v]
Then, inside the Gobo sprite:
when I receive [Gobo 1 v] say [how are you?] for (2) secs broadcast [Scratch Cat 2 v]
The two sprites continuously broadcast back and forth to each other to say a message. However, when the conversation becomes long, multiple broadcasts are needed and the scripts can become unorganized.
Broadcast and Wait method
This method is slightly more efficient, because you only need one script in the first sprite to talk.
when gf clicked //in the first sprite say [Hello!] for (2) secs broadcast [Gobo 1 v] and wait say [How are you today?] for (2.5) secs broadcast [Gobo 2 v] and wait //and so on
Then, inside of Gobo:
when I receive [Gobo 1 v] say [Hi there!] when I receive [Gobo 2 v] say [Very fine indeed! And you?]
The variable method uses a variable to trigger the other sprite to talk. After a sprite finishes speaking, it changes a variable by "1", in which the other sprite recognizes and then begins speaking. The following script would go into "Scratch Cat".
when gf clicked set [talker v] to  //sets it so the conversation starts from the beginning and functions properly say [Hi Gobo] for (2) secs change [talker v] by (1) //signals Gobo to talk wait until <(talker) = > //wait until Gobo is done responding say [I am good, how about you?] for (4) secs change [talker v] by (1)
And for the "Gobo" sprite:
when gf clicked wait until <(talker) = > //wait until Scratch Cat greets say [Hi Scratch Cat. How are you?] for (3) secs change [talker v] by (1) //signals Scratch Cat to talk wait until <(talker) = > say [I am great also!] for (3) secs
The script can keep cycling continuously. The script can become very long from a long conversation, but it doesn't use multiple of any broadcasts. The variable method is often best for unordered conversations, in which there are multiple sprites not speaking to one another in a patterned fashion.
The list method uses the same amount of scripting no matter how long the conversation is. It is a great method to reduce large scripts and a project's file size. The list method uses two scripts, one of "Scratch Cat's" messages and one of "Gobo's". The first sprite says a message, triggers the second sprite to speak, and then the cycle continues, with a variable being changed after both have fully spoken, to signal the next message from the lists. The lists are used to simply store the text which each sprite will speak, and is ordered within. The following script goes into "Scratch Cat"
when gf clicked set [item# v] to  //so the first message is the first item in the list of messages set [talker v] to [Scratch Cat] //to know that "Scratch Cat" speaks first forever say (item (item#) of [Scratch Cat messages v]) for (3) secs //says the current item from the list of messages set [talker v] to [Gobo] //signal "Gobo" to speak wait until <(talker) = [Scratch Cat]> //wait until it's its turn to speak
And for "Gobo":
when gf clicked forever wait until <(talker) = [Gobo]> //wait until it's its turn to talk say (item (item#) of [Gobo messages v]) for (3) secs //say the item in the list of messages change [item# v] by (1) //so the next message in the lists is said by each sprite instead of the same one set [talker v] to [Scratch Cat] //signals "Scratch Cat" to speak