I'm not really much of one for a bait and switch, holding an ace up my sleeve, or pulling any sort of trickery on a candidate. It doesn't seem to foster a good relationship and tends to cause trust issues. For this very reason I provide as much information as I possibly can while maintaining a level playing field - I just happen to be pretty particular about my timing.
There are a lot of things a candidate should know prior to an interview. He or she should have a good idea of the location, the facility, the environment, the job duties, the team size, and the reporting structure. There may even be a few more here and there, but those basics are usually enough information for a person to have interest in a job and generate the desire to have it. The thing is, they don't need any of that information to convince me whether or not they are qualified for said job.
In a perfect world I would have all the freedom to determine what kind of job postings I put out. If this were the case the job description would be relatively short and bare-bones, would include the mandatory minimum qualifications, and a lot of information about why my company is great to work for. My goal is to have the candidate very excited about working for my company and this should be the main focus of the initial conversation. If the benefits of working for a company are great enough, the specific role within said company may be less important and then the recruiter is free to do some front-end qualifying of a candidate without a job-specific bias.
This is sometimes more difficult to do with a candidate who has seen a job posting, especially if it provides detailed information about the job at hand. They will be armed with a relatively stock lecture of their related experience and why they should be granted an interview.
Very bland, very boring, very useless.
Ok, so I admit, not entirely useless, but not entirely useful either. This is why I love doing my own sourcing. When I make contact with prospective candidates it is a very good possibility they have not read the job posting, have no idea what job I am talking about, and the conversation is free to be more general. Although there may be a specific position in mind, the discussion can be initiated under the guise of general interest in the corporation, or even a segment or group therein. Some information should be given out at this point to generate that interest, and then the recruiter can begin asking about the candidate's background to assess their general profile. During the course of this discussion it should become clear how much or how little experience the candidate actually has in the areas needed for the position to be filled. A strategic use of probing questions will ensure that you get more information on certain subjects when needed without fully tipping your hand or 'leading the interview.'
Once you've done a full review of the candidate there are a couple of routes that can be taken. One is to state that you will review their background with your current openings to determine where they can best be utilized and will contact them with potential fits. The other is to immediately transition into a discussion about the specific role you are looking to fill. The approach will depend on the type of candidate you have on the line.
This is the point at which I will 'give up the goods' so to speak regarding the opportunity that is available. I already know that the candidate wants to work for my company and that he or she is qualified for the job at hand. I should also have a good idea whether or not the level of the position is appropriate if I did a good job during the initial screening. This makes the 'selling' of the job much easier and should be a very exciting conversation for the candidate. I also engage in more in-depth discussions about the candidate's job specific qualifications at this point. This is a very natural transition and candidates are generally happy to expound a bit more on the experience which relates to a specific opportunity.
And why is that?
Why would they be lacking information or why can the conversation be more general?
I think the lack of info part is pretty simple since they have to visit a specific site or view a job posting in order to know I am looking to fill a position and the applicable details, but I think you're asking about the general nature of the conversation...
Limiting how much the candidate knows on the front end means they are less likely to filter and tailor the information they give out about their experience and background. Let's say for example that I'm looking for an engineer to work on door structures and assemblies. If the candidate knows this up front it may be like pulling eye teeth to get information about other experiences out of him or her. They know what the job entails and will be inclined to beef up the experience they have in that area. If the only thing they are aware of is that my company designs, engineers, and manufactures vehicles they will be forced to give a more broad overview of their experience. If they spend a good deal of time discussing body structures, doors, moving mechanisms, etc. my level of confidence goes up that they may have the skills to perform this job. However, if they go really in-depth about a bunch of unrelated experience like seating or airbags and barely touch on doors they may be better qualified for a different department.