Welcome - there is great a variety of experiences to be explored. The best thing is that there is really nothing to lose - try all the different means of causing your diaper to fill. With the passage of time, passage of stool is inevitable. Many others have already posted various means of fiber loading" leading to impressive unloading.
Oral laxatives give different kinds of effects. Osmotic (saline & glycerine, magnesium based products, to name a few) have a short fuse - sometimes less than an hour, but the result is quite watery. They work by drawing water from the body into the intestine, causing in inevitable flood. Volume and pressure make it impossible to hold back, but the watery result is often hard for the diaper to contain, and doesn't have the squishy soft adhesive texture that many of us crave.
Castor oil is unpredictable, working in anything from 45 minutes to 6 hours, although 2 - 5 hours seems to be the most frequent range of time. For me, it takes at least 3 - 4 hours. The result is soft, sudden, and often quite urgent. If you like suspense, and a huge mess, this is for you, since it cleans out the whole intestinal system, to the point that it will be a few days until you need to poop again. The downside is that many people find it very obnoxious to take, it has a bad aftertaste and an oily feeling in the mouth etc. I have found that many users have found ways of disguising it with fruit juices or chocolate milk. Once you get it down, a swig of wine will dissolve the oil, and leave your mouth feeling refreshed. Castor oil doesn't seem to cause as much electrolyte imbalance as the osmotics do. It is best for certainty that something will happen, and no way to know when. Soft, sticky, often overwhelming quantity, unpredictable timing. If you're looking for adventure, this is the one, if you can get it down. (I just take it from the spoon & rinse with wine). This is my personal favorite. Hold back a day or two, with Immodium AD to prevent "going" the day before, and take it before going to bed. I find it difficult to sleep, with a sense of nervous anticipation, but I have always been rewarded in the end. Some have used alcohol and/or sleep aids, from melatonin all the way to prescription sleeping pills, to be sure that they wake up either after the floodgates open, or just as they are opening, with no chance to react. Everything pushes out with an involuntary spasm and a desperate grunt, followed by an immense sense of comfort and relief, only to have the process repeat itself at least 2 or 3 more times. Being drunk while this is happening adds a whole new level of sensation, but be careful. Some people find the castor oil nauseating, and the alcohol would push them over the edge to pure misery. Getting sick is a definite downer. The usual dose recommendation is 1 - 4 tablespoons; start low & build up until you find the "sweet spot" for you. I have found the higher range, or even an extra tablespoon or 2, is best for a satisfying, explosive result.
Castor oil is classified as a stimulant laxative, but it works a bit differently than the others in this class. It lubricates and softens, but it doesn't liquefy like the osmotics do. Only at the end of the process, when the small intestine is emptying out, do things get runny. Others have posted on the technical differences. Stimulant laxatives irritate the large intestine, causing a lot of movement, often cramping, and finally a desperate urge. There is no softening or change in texture. Some people combine stimulants with 1/2 the recommended dose of an osmotic, to loosen things up before pushing them out.
There are also suppositories and enemas; if you have a short period of time, and want to know when things will happen, they are very effective - sudden, urgent, and unstoppable. Enemas often have the problem of too much water, again "spoiling" the consistence of the result. There is one recipe posted on this site that I want to try as soon as the liquid glycerine I ordered comes in. It is in this section, under the heading "Solution for an uncontrollable urge". It seems to be a great way to a quick and satisfying result.
Something I just heard about and haven't tried is something called "J lube". It is a powder that becomes a very persistent lubricant when it gets wet. Supposedly, when it is sprinkled in the front part of the diaper, every release of urine produces an exquisite goo that provides exquisite stimulation of the genital area, male or female. I will definitely try it, and report to the community on the results. A good side effect is that it makes the pee less liquid, reducing leakage.
Whatever you end up doing, have fun and be safe. Start on the lower end of the dosage recommendations. Heed the warnings, and don't indulge in chemical aids too often. Protect your skin with petroleum jelly or zinc diaper creams, especially if you want to keep the filled diaper on. That also reduces the smell that can penetrate the skin. Have ointments on hand should a rash develop. And know that you are part of a large community of friends that will be glad to share your experiences.