"Actionable next steps" depend on the situation -- informational interview next steps are very different than job interview next steps, but generally, the idea is to create an email thread of follow ups.
For instance, if you’ve just gone on an informational interview, perhaps you heard about a department at your new contact’s company that you’d like to learn more about. Maybe the person even offered to make an introduction during the meeting. If so, take him up on the offer! A thank you note is your time to remind your contact to actually follow through on the promises he made in person. If you forget to add a reminder, the other person will likely forget about the request entirely -- or worse, remember and decide that if it's not important enough for you to remember, it's not important enough for him to waste his time and social capital.
But maybe an intro or favor isn’t exactly what you’re looking for, and you're simply hoping to grow the relationship. You can still create a thread that warrants a response by calling back to information discussed in the meeting. For example, you could send a link to an interesting article that had been mentioned or watch a show/read a script the person recommended and offer your insights. This may not earn you an immediate result, but in the long term, it will help you build a rapport with the person you met with and encourage a lasting connection.
After an actual job interview, your actionable next steps will be to offer to send referencesor any other material the person needs. You also have an additional opportunity to ask about the hiring timeline if it didn’t come up during the meeting. Ultimately, the idea is to keep the conversation going and show that you are not only interested in the position, but are willing to be proactive to obtain it.
Thank you notes are great for opening up lines of email communication that will help a person remember you and allow for regular follow ups. Make sure you take advantage of this opportunity!