Sticking to the script at the same time as listening and conversing is a must. 

A well thought out and prepared script means that you know how the conversation should play out and helps you to focus on the objective or result you want to achieve.

Having the ability to think on your feet and adlib a little is always a bonus,  but following the basic outline of the script will help to keep the conversation flowing and heading in the desired direction.

If they inquire about something off topic – acknowledge it – answer the question as best as you can, then bring them back to the main reason for your call.

Following a script will guarantee all appropriate information about your product or service has been given to the prospect and that no vital details have been missed, it also saves you the embarrassment of repeating yourself or losing your train of thought.