Before you even write a single line of code, it's important to write a functional specification so the development team has a clear idea of what the bot is expected to do. The specification should include a reasonably comprehensive list of user inputs and expected bot responses in various knowledge domains. This living document will be an invaluable guide for developing and testing your bot.
Having a conversation with a computer might have seemed like science fiction even a few years ago. But now, most of us already use chatbots for a variety of tasks. For example, as end users, we ask the virtual assistant on our smartphones to find a local restaurant and provide directions. Or, we use an online banking chatbot for help with a loan application.
As the above chart (source) illustrates, email click-rate has been steadily declining. Whilst open rates seem to be increasing - largely driven by mobile - the actual engagement from email is nosediving. Not only that, but it's becoming more and more difficult to even reach someone's email inbox; Google's move to separate out promotional emails into their 'promotions' tab and increasing problems of email deliverability have been top reasons behind this.
This is the big one. We worked with one particular large publisher (can’t name names unfortunately, but hundreds of thousands of users) in two phases. We initially released a test phase that was sort of a “catch all”. Anyone could message a broad keyword to their bot and start a campaign. Although we had a huge number of users come in, engagement was relatively average (87% open rate and 27.05% click-through rate average over the course of the test). Drop off here was fairly high, about 3.14% of users had unsubscribed by the end of the test.
Some brands already seem to be getting the balance right. A bot needs to capture a user's attention quickly and display a healthy curiosity about their new acquaintance, but too much curiosity can easily push them into creepy territory and turn people off. They have to display more than a basic knowledge of human conversational patterns, but they can't claim to be an actual human -- again, let's keep things from getting too creepy here.

I know what you’re thinking – when will the world of marketing just stand still for a moment and let us all catch up?!?! No such luck, dear readers. No sooner have we all gotten to grips with the fact that we’re going to have to start building live video campaigns into our content marketing strategies, something else comes along that promises to be the next game-changer. And so here we are with the most recent marketing phenomenon – chatbots.

Regardless of which type of classifier is used, the end-result is a response. Like a music box, there can be additional “movements” associated with the machinery. A response can make use of external information (like weather, a sports score, a web lookup, etc.) but this isn’t specific to chatbots, it’s just additional code. A response may reference specific “parts of speech” in the sentence, for example: a proper noun. Also the response (for an intent) can use conditional logic to provide different responses depending on the “state” of the conversation, this can be a random selection (to insert some ‘natural’ feeling).
ELIZA's key method of operation (copied by chatbot designers ever since) involves the recognition of clue words or phrases in the input, and the output of corresponding pre-prepared or pre-programmed responses that can move the conversation forward in an apparently meaningful way (e.g. by responding to any input that contains the word 'MOTHER' with 'TELL ME MORE ABOUT YOUR FAMILY').[9] Thus an illusion of understanding is generated, even though the processing involved has been merely superficial. ELIZA showed that such an illusion is surprisingly easy to generate, because human judges are so ready to give the benefit of the doubt when conversational responses are capable of being interpreted as "intelligent".